A well-known process for the production of acetic anhydride from acetic acid involves the formation of ketene (ethenone). Thereby ketene is produced by dehydrating acetic acid at high temperatures (typically in a ketene furnace operated at temperatures of the order of 700° C. to 750° C.). Subsequently, the ketene is reacted with acetic acid in an exothermic reaction leading to the formation of acetic anhydride.
Interestingly, both acetic acid and acetic anhydride are used in processes for the acetylation of wood. These processes, for which there is an increasing demand, serve to provide the wood with improved material properties, e.g. dimensional stability, hardness, durability, etc. In these processes, excess acetylation medium, typically a mixture of acetic anhydride and acetic acid, is ultimately removed from the wood. It is thereby desired to avoid wasting the removed acetylation medium, and preferably to recirculate and re-use it in wood acetylation.
Suitable techniques exist for recovering acetic anhydride, by separating it from acetic acid, after which the acetic anhydride can be re-used in wood acetylation. The acetic acid, however, comes in an excess ratio after wood acetylation as it is formed as a byproduct thereof, and it would be desired to put this to separate use, sell it as a chemical, and/or use it in the production of ketene. However, the specific source of the acetic acid, viz. from the acetylation of wood, comes with inherent limitations to their further use due to the presence impurities such as that of terpenes and terpenoid impurities from the wood. Particularly terpenes and terpenoids are difficult to remove. This limits the use of acetic acid as recovered from wood acetylation. E.g., using it in a ketene furnace is not desired, as the aforementioned impurities are prone to result in coke formation in the furnace, as a result of the high temperatures applied therein.
The foregoing issue is addressed in WO 2009/120257, by azeotropic distillation, wherein acetic acid comprising the aforementioned impurities is supplied to a distillation column together with water. Whilst the reference thus teaches a method of obtaining purified acetic acid from wood acetylation, it does not relate to the production of acetic anhydride and, particularly, it does not teach how to effectively integrate the production of acetic anhydride and the acetylation of wood. Also, the addition of water to the distillation column reduces the economic feasibility of the process. Further, the method would require investment costs and operational costs associated with the addition of a distillation unit.
It is desired to provide a method by which the acetylation of wood and the production of acetic anhydride can be effectively integrated. Also, it is desired to thereby make optimal use of sources of liquid as available from wood acetylation.